1. Field of the Invention
The present invention relates to a method of analyzing wavelength-division multiplexed signal light. More particularly, it relates to a method and an apparatus for analyzing wavelength-division multiplexed signal light which feature the capability of determining reference wavelengths automatically.
2. Description of the Related Art
In optical communications where information signals are carried on intensity-modulated light, the use of wavelength-division multiplexing (WDM) is rapidly increasing today. In WDM, light output from n light sources of different wavelengths (hence, frequencies) is multiplexed so that optical signals having different wavelengths (hence, frequencies) are treated as separate channels.
FIG. 1 shows an exemplary optical spectrum of wavelength-division multiplexed signal light. The wavelength (hence, frequency) of each channel of this light is customarily of any value that is selected from among reference wavelengths xcexC1, xcexC2, . . . xcexCm spaced at equal intervals as specified by the ITU-T (International Telecommunication Union-Telecommunication sector) standards. In FIG. 1, xcexAn is the reference wavelength of channel n as selected from among those standard reference wavelengths. The optical intensities of the respective channels are represented by P1, P2, . . . Pn and it is usually considered ideal that they have no variations but are constant.
FIG. 2 shows an exemplary optical spectrum of actual wavelength-division multiplexed signal light having n channels. In the optical spectrum shown in FIG. 2, the central wavelength xcexBn of each channel contains an error xcex94xcexn with respect to the reference wavelength xcexAn specified by the ITU-T standards and the optical intensity Pn of each channel also contains an error xcex94Pn. In optical communications using wavelength-division multiplexed signal light, interference will occur if the errors xcex94xcexn and xcex94Pn are significant in each channel.
Therefore, in the actual use of wavelength-division multiplexed signal light in optical communications, the errors xcex94xcexn and xcex94Pn in the optical spectrum of the light are usually subjected to preliminary analysis, typically using an optical spectrum analyzer that measures intensity vs. wavelength characteristics by means of a spectrometer having in its interior a diffraction grating or some other device to separate the incident light into its spectral components.
FIG. 3 illustrates how the spectrum waveform of a single channel n in the wavelength-division multiplexed signal light is analyzed. To analyze the central wavelength xcexBn and the peak power Pn, the spectrum waveform""s peak level Pn is determined within the range of xc2x1xcex94xcexR from the reference wavelength xcexAn of the channel of interest; then, a line indicating Pn minus a threshold level TH (=Pnxe2x88x92TH) is drawn parallel to the horizontal axis of the graph in FIG. 3; since the line for Pnxe2x88x92TH crosses the spectrum waveform at two points, the wavelength of the midpoint between the two crossing points is determined as the central wavelength xcexBn of the channel of interest; the difference between the reference wavelength xcexAn and the central wavelength xcexBn is taken to determine the wavelength error xcex94xcexn.
In order to analyze optical spectra by the above-described method, the reference wavelength xcexAn must preliminarily be known for each channel and the value of xcexAn for each channel must be registered in a memory before analysis starts.
FIG. 4 shows an exemplary optical spectrum of 4-channel wavelength-division multiplexed signal light. In the illustrated case, four of the reference wavelengths xcexCm as determined by the ITU-T standards are allotted as the reference wavelengths xcexAn of the four channels and they are xcexC1, xcexC3, xcexC5 and xcexC7 corresponding to xcexA1, xcexA2, xcexA3 and xcexA4, respectively, which must be registered in the memory before actual analysis of the spectrum starts.
FIG. 5 shows another exemplary optical spectrum of 4-channel wavelength-division multiplexed signal light. In the illustrated case, four of the reference wavelengths xcexCm as determined by the ITU-T standards are allotted as the reference wavelengths xcexAn of the four channels and they are xcexC1, xcexC2, xcexC5 and xcexC6 corresponding to xcexA1, xcexA2, xcexA3 and xcexA4, respectively, which must be registered in the memory before actual analysis of the spectrum starts.
Thus, prior to analyzing optical spectra, the reference wavelength xcexAn of each channel in the spectrum to be analyzed need be set in accordance with the channel wavelength xcexCm of the spectrum.
There may be a case where analysis of an optical spectrum comprising one combination of channels, say, the spectrum shown in FIG. 4, is followed by analysis of another optical spectrum comprising a different combination of channels, say, the spectrum shown in FIG. 5. In a case like this, the reference channel xcexAn of each channel must be reset in accordance with the channel wavelengths xcexCm of the spectrum under analysis.
The above-described prior art method may be used to analyze n-channeled wavelength-division multiplexed signal light in accordance with the flowsheet shown in FIG. 6.
First, reference wavelengths xcexA1, xcexA2, . . . xcexAn corresponding to the respective channels of the wavelength-division multiplexed signal light to be analyzed are input and stored in a memory (step A1).
Then, the optical spectrum of the wavelength-division multiplexed signal light is measured with an optical spectrum analyzer or any other suitable apparatus and the measured spectrum waveform data is stored in the memory (step A2).
Then, on the basis of the measured waveform data, the spectrum waveform""s peak level Pn is determined within the range of xc2x1xcex94xcexR from the reference wavelength xcexAn of each channel; a line indicating Pn minus a threshold level TH (=Pnxe2x88x92TH) is drawn parallel to the horizontal axis of the spectrum graph; since the line for Pnxe2x88x92TH crosses the spectrum waveform at two points, the wavelength of the midpoint between the two crossing points is determined as the central wavelength xcexBn of each channel (step A3).
Subsequently, the difference between the reference wavelength xcexAn and the central wavelength xcexBn is taken to determine the wavelength error xcex94xcexn (step A4).
Then, the central wavelength xcexBn and the wavelength error xcex94xcexn are output to a display or some other device as the results of analysis (step A5).
If the analysis is not to be repeated, the process ends but if the user wants another analysis, the process continues (step A6).
If the wavelength-division multiplexed signal light to be analyzed varies in channel wavelength xcexAn, the sequence returns to the step of inputting another set of reference wavelengths xcexA1, xcexA2, . . . xcexAn corresponding to the respective channels of the wavelength-division multiplexed signal light (step A1); if there is no such change, the sequence returns to step A2.
As will be understood from the foregoing description, the prior art method of analyzing wavelength-division multiplexed signal light has the disadvantage of time-consuming input step since the reference wavelengths xcexAn corresponding to the respective channels must be input one by one before analysis starts.
The same disadvantage occurs if the combination of channels in the optical spectrum to be analyzed is different from the previous one and this is because the reference wavelengths xcexAn that correspond to the respective channels in the optical spectrum under current analysis have to be reset one by one.
An object of the present invention is to ensure that the data about the reference wavelengths xcexAn which are necessary to analyze the optical spectrum of wavelength-division multiplexed signal light are determined automatically from the spectrum waveform data for the optical spectrum of actual wavelength-division multiplexed signal light and stored in a memory, thereby eliminating the need to input the reference wavelengths xcexAn one by one even in the case where the combination of channels in the optical spectrum to be analyzed is different from the previous one.
To attain this object, the apparatus of the invention for analyzing wavelength-division multiplexed signal light performs the following steps in sequence:
storing a plurality of reference wavelengths (xcexC1-xcexCm) in a memory as preliminary specified by the ITU-T standards;
determining maximum points of light intensity from spectrum waveform data obtained by measuring an actual wavelength-division multiplexed optical spectrum, identifying as a channel any one of the maximum points which at least differs in light intensity from the two minimum points, one being right to said maximum point and the other being left, by at least a channel identifying threshold (TH) level, and determining the wavelength of the identified channel as xcexDn;
then rounding the xcexDn for each channel to the value of the nearest ITU-T grid wavelength xcexCm so as to determine the reference wavelength xcexAn for each channel and storing the determined reference wavelengths xcexAn in the memory.
As a result, the reference wavelengths xcexAn can be determined automatically from the actual optical spectrum of wavelength-division multiplexed signal light. This eliminates the need to input the reference wavelengths xcexAn one by one and even if the combination of channels in the optical spectrum to be analyzed is different from the previous one, the reference wavelengths xcexAn can be easily determined and stored in the memory.
The method of the invention for analyzing wavelength-division multiplexed signal light attains the above-mentioned object by adopting the design set forth in one of the following paragraphs 1-8:
1. Analyzing the wavelength (xcexDn) of each channel from input data for optical spectrum waveform, storing the analyzed wavelength (xcexDn) of each channel as the reference wavelength (xcexAn) of each channel, and analyzing the central wavelength (xcexBn) of each channel from the input data for optical spectrum waveform and the reference wavelength (xcexAn) of each channel.
2. Storing reference wavelengths (xcexCm) in a memory as preliminary specified by the ITU-T standards, analyzing the wavelength (xcexDn) of each channel from input data for optical spectrum waveform, rounding the analyzed wavelength (xcexDn) of each channel to the value of the nearest reference wavelength (xcexCm) so as to analyze the reference wavelength (xcexAn) of each channel, storing the analyzed reference wavelength (xcexAn) of each channel, and analyzing the central wavelength (xcexBn) of each channel from the input data for optical spectrum waveform and the reference wavelength (xcexAn) of each channel.
3. Storing a plurality of reference wavelengths (xcexC1- xcexCm) in a memory as preliminary specified by the ITU-T standards, determining maximum points of light intensity from spectrum waveform data obtained by measuring an actual wavelength-division multiplexed optical spectrum, identifying as a channel any one of the maximum points which at least differs in light intensity from the two minimum points, one being right to said maximum point and the other being left, by at least a channel identifying threshold (TH) level, determining the wavelength of the identified channel as xcexDn, then rounding the xcexDn for each channel to the value of the nearest ITU-T grid wavelength xcexCm so as to determine the reference wavelength xcexAn for each channel, storing the reference wavelengths xcexAn in the memory, thereby determining the reference wavelengths xcexAn automatically from the actual wavelength-division multiplexed signal light.
4. A first storage step for storing reference wavelengths (xcexCm) in a memory as specified by the ITU-T standards;
a second storage step for measuring the optical spectrum of the wavelength-division multiplexed signal light to be analyzed and storing the measured spectrum waveform data in the memory;
the step of determining the wavelength (xcexDn) at the maximum point of each channel from the waveform data;
a third storage step for rounding the wavelength (xcexDn) at said maximum point to the value of the nearest reference wavelength (xcexCm) and storing the rounded value in the memory as a reference wavelength (xcexAn);
a central wavelength analyzing step for determining the central wavelength (xcexBn) of each channel from the spectrum waveform data and said reference wavelength (xcexAn);
a first step for determining a wavelength error (xcex94xcexn) which is the difference between said central wavelength (xcexBn) and said reference wavelength (xcexAn); and
an output step for outputting said central wavelength (xcexBn) and the wavelength error (xcex94xcex) as the results of analysis.
5. Including an additional step between said step for determining the wavelength (xcexDn) at the maximum point of each channel from the waveform data and said third storage step, said additional step being for identifying as a channel any one of the maximum points which at least differs in light intensity from the two minimum points, one being right said maximum point and the other being left, by at least a channel identifying threshold (TH) level and determining the wavelength (xcexDn) of that maximum point.
6. Including a reference wavelength resetting step between said second storage step and said step of determining the wavelength (xcexDn) at the maximum point of each channel, said additional step being for determining if it is necessary to reset the values of the reference wavelengths (xcexAn) and allowing the sequence to proceed to said central wavelength analyzing step if no resetting of the reference wavelengths (xcexAn) is made in said reference wavelength resetting step.
7. If it has been decided in said reference wavelength resetting step to reset the values of the reference wavelengths (xcexAn), the sequence proceeds to the step of determining the wavelength (xcexDn) at the maximum point of each channel from the waveform data.
8. The output step for outputting the central wavelength (xcexBn) and the wavelength error (xcex94xcex) as the results of analysis is followed by the step of determining as to whether a repeated measurement is to be made and if the answer is yes, the sequence returns to the second storage step for measuring the optical spectrum of the wavelength-division multiplexed signal light to be analyzed and storing the measured spectrum waveform data in the memory.
The apparatus of the invention for analyzing wavelength-division multiplexed signal light attains the above-mentioned object by adopting the design set forth in one of the following paragraphs 9 and 10:
9. Comprising:
a first analysis means for analyzing the wavelength (xcexDn) of each channel from input data for optical spectrum waveform;
a first storage means for storing the analyzed wavelength (xcexDn) of each channel as the reference wavelength (xcexAn) of each channel; and
a second analysis means for analyzing the central wavelength (xcexBn) of each channel from the input data for optical spectrum waveform and the reference wavelength (xcexAn) of each channel.
10. Comprising:
a second storage means for storing reference wavelengths (xcexCm) in a memory as preliminary specified by the ITU-T standards;
a first analysis means for analyzing the wavelength (xcexDn) of each channel from input data for optical spectrum waveform;
a third analysis means for rounding the analyzed wavelength (xcexDn) of each channel to the value of the nearest reference wavelength (xcexCm) so as to analyze the reference wavelength (xcexAn) of each channel;
a third storage means for storing the analyzed reference wavelength (xcexAn) of each channel; and
a second analysis means for analyzing the central wavelength (xcexBn) of each channel from the input data for optical spectrum waveform and the reference wavelength (xcexAn) of each channel.